A mixer circuit (sometimes referred to as a "modulator") provides an output signal at a frequency or frequencies which are dependent upon the frequencies of two or more input signals.
A typical wide-band double-balanced mixer has two matched differential pairs of transistors which are biased by an appropriate biasing circuit. These differential pairs are base-coupled to a local oscillator input and are emitter-coupled to the two transistors of a third differential pair. This pair is base-coupled to a radio frequency input. Typically the output is taken from the collectors of the two initial differential pairs, either single-ended or differential.
It has been observed in balanced active mixing circuits that there exists a significant noise degradation within the circuit of typically 16 decibels (dB). Noise reduction in mixing circuits is a frequently sought-after but frequently elusive goal.
Since many present high frequency radio communication systems are narrow-band in nature, it would be preferable for these applications to trade off the wide-band and high noise degradation characteristics of the typical active mixer circuit for a circuit topology which maintains all the characteristics of a conventional active mixer in the frequency band of interest but which is capable of delivering the desired performance with a significantly reduced level of undesirable noise.